Punching, piercing, perforating, stamping and broaching are conventional mechanical operations in which a punch, die punch or other penetrating member passes into a workpiece. Upon withdrawal of the piercing member, more often than not there is a tendency for the workpiece to adhere to the forming machine element. This tendency has been known to dimple or otherwise warp or distort the workpiece, especially if the workpiece is of thin gauge. Accordingly, it has been conventional to provide stripping devices such as metallic helical or elastomeric spring members with flat caps or stripper plates in association with the punch to push the workpiece off of the punch nib as the punch is withdrawn therefrom.
The designs of conventional prior art strippers have had their drawbacks, however. Stripper plates must be specially designed and machined for each particular job resulting in excessive cost and inflexibility from the point of view of modification once the job has been set up and the stripper plate machined. A further difficulty with stripper plates is that the cross sectional shape of the punch and observance of manufacturing tolerances may preclude fabrication of the plate with a punch passage of the same shape and size as the punch so that the stripper plate does not provide stripping pressure to the workpiece immediately adjacent to all sides of the punch. For thin workpieces, this may result in an undesireable slightly raised burr or dimple distortion when the workpiece is stripped from the punch.
On the other hand, these and other difficulties with the use of stripper plates have been surmounted by elastomeric spring members with ends surrounding each punch as exemplified in U.S. Pat. No. 2,805,717 and especially in U.S. Pat. No. 3,690,205. Such elastomeric strippers, however, are not suitable for punching operations close to an edge of the workpiece or close to a surface discontinuity such as a rib or groove. In such an application, the elastomeric ends are subject to uneven deformation, stress concentration, and failure when repeatedly pressed against the edge or discontinuity. Furthermore, while the punch passage through the elastomeric ends may be made of closely conform to even the most complex punch cross-sections by actually punching the passage through the elastomeric material during an initial operation, the stripping pressure is distributed across the entire flat face of the stripper in such a manner that the stripping pressure immediately adjacent to the punch nib is somewhat less than the pressure exerted by the elastomeric spring member itself so that it may be seen that the stripping pressure exerted by the elastomeric stripper increases slightly in all directions away from the punch.